Syntha-6 or whey and casein?

I am 15 and looking to gain serious mass of muscle. I go to the gym a lot and eat right but I gotta get more. I'm either gonna buy a big tub of gold standard whey and gold standard casein (85 bucks total and it lasts 55 days) or a tub of syntha-6 for 40 bucks to last 55 days. What's better and why?